Stuttering is a speech deficiency which is characterised by certain interruptions or other voice problems in the speech flow, such as repetitions, prolongations and unintentional pauses in the speech. Approximately 1% of the adult population and 4% of the children suffer from stuttering. The reason for stuttering has not been fully explained, but it is probably caused by several different and interrelated factors. In one of the theories it is suggested that there is a conflict between the two hemispheres of the brain in such a way that the first, dominant hemisphere is disturbed by the second hemisphere of the brain. Then the brain signals to the muscles that produce speech is impaired so that stuttering is caused.
There are several different methods for the treatment of stuttering. Some of these methods might have been succesful for a short period of time, but they do not provide a satisfactory solution to the problem of stuttering. Modern speech therapy includes methods of psychological nature, such as methods for reducing the stutterers fearness for stuttering and avoidance but also methods for improving the stutterers self-confidence have been used. In main hospitals medically trained speech therapists are usually engaged in the treatment of speech and voice problems. Stuttering therapy is a hard and often arduous work, and it is certainly only a work in the long run.
It has been a main feature in almost all of these prior anti-stuttering methods to decrease the speech flow, but then hard training is required. For that reason there are different devices on the market which can be used for training slow speech. These devices are most often based on some type of auditory feedback by means of a microphone and headphones and they can be divided into three main categories:
Delayed Auditory Feedback (DAF)
Delayed auditory feedback (DAF) is based on a delay of the user's voice a fraction of a second between the microphone and the headphones. Typical delays are in the range of 50-200 ms. According to the theory this discrepancy is sensed by the brain so that the brain provides additional resources to the speech center resulting in a more fluent speech flow.
Frequency-Shifted Auditory Feedback (FAF)
Frequency-shifted auditory feedback (FAF) is based on a change of the frequency characteristics of the user's voice in the headphones so that the speech which is heard by the user is somewhat distorted. A typical frequency shift could be around a half octave. It is supposed that the speech center in the brain is activated in a similar way as for the delayed auditory feedback.
Masked Auditory Feedback (MAF)
According to the so-called masked auditory feedback (MAF) a synthetic sine signal is generated in the headphones. The frequency of this sine signal should correspond to the user's own phonetical frequency. In case of stuttering problems the signal is activated by the user himself and it is supposed that this signal then activates the speech center of the brain.
The most frequently used anti-stuttering devices are all based on delayed auditory feedback and these devices will therefore be described some more in detail here. These devices are supposed to be portable and they all comprise some type of headphones or earpieces in order to reproduce the voice of the user with some delay.
One example of such a device which comprises two headphones and at least one microphone which are connected to an amplifier is described in SE 427418. This device comprises a delay circuit in which the signal from the amplifier to one of the headphones is delayed compared to the other signal. This other, undelayed signal which is transmitted to one of the user's ears is sensed earlier by the dominant half of the brain so that the other half of the brain will not be able to induce stuttering according to the theory described in the patent.
The headphones are preferably made for permanent use and comprises a set of miniaturised earpieces of the type which can be inserted into the ear canal. The earpieces and the microphone are preferably connected into a single, portable unit for the user depending on his age and the intensity of his stuttering problems.
Other examples of anti-stuttering devices which are based on the delayed auditory feedback principle are disclosed in U.S. Pat. Nos. 3,566,858 4,685,448 5,478,304 5,961,443 and EP patent 1 110 519. These patents describe different solutions for the delay and frequency shift as well as techniques for picking up more of the user's voice compared to the surrounding sound.
A disadvantage with the described anti-stuttering devices is the fact that they are blocking the ear canal of the user. This in turn depends on the fact that the auditory-feedback is based on the principle that the sound is amplified and fed into the auditory meatus and stimulates the eardrum from the outside, i e a traditional air-conducting hearing aid principle. In order to prevent acoustic feedback problems in these devices, the auditory meatus is almost completely plugged by a hearing plug or by the hearing aid device itself. This causes the user a feeling of pressure, discomfort, and sometimes even eczema. In some cases it even causes the user problems like running ears due to chronic ear inflammations or infections in the auditory canal.
Another disadvantage with this type of device is the fact that it must be removed when it is not used as an anti-stuttering aid. If the device should be remained in the auditory meatus when the person is not talking, but only listening, it is blocking sound from the surroundings. However, a person with stuttering problems has normally no hearing problems and he does not need the device as a hearing aid in this case.